1. Field of the Invention
This invention relates to a tool holder for holding a straight shank of a cutting tool or a tool adapter, which straight shank has flat surfaces on part of the outer peripheral surface thereof.
2. Description of the Prior Arts
Generally, a cutting tool employs a straight shank provided with flat surfaces along the secant of the outer circumferential circle of the shank. The prior art tool holder for holding a straight shank with flat surfaces has been such that set-screws are driven into the wall of the holder at a right angle with respect to the flat surfaces of the straight shank inserted in the holder, in a manner that the tips of set-screw stems are brought into engagement with the flat surfaces of the shank directly, whereby the shank is locked to the holder.
In the prior art tool holder, each set-screw functions as a cantilever, so that the shank-locking force is sustained by the tip of each set-screw, and the locking surface by which the shank is locked to the holder is confined only to the tip of the set-screw. Because of such mechanism, the tip face of each set-screw suffers deformation, which eventually develops into damage in a threaded portion, when that force acts on the tip of set-screw transversely thereof, which force originates from an unusual or greatly strong cutting force at the cutting operation. Furthermore, since each set-screw directly engages each flat surface of the shank, vibration arising at the cutting operations is directly transmitted to the tip face of each set-screw, with a likelihood that engagement of the tip of each set-screw with each flat surface of the shank becomes released.